Organic Light-Emitting Devices (OLEDs) typically encompass display devices that sandwich carbon-based films between two charged electrodes, one a metallic cathode and one a transparent anode, often glass. The films include a hole-injection layer, a hole-transport layer (HTL), an emissive layer (EL) and an electron-transport layer (ETL). If voltage is applied to the OLED cell, injected positive (holes) and negative (electron) charges may recombine in the emissive layer and create electroluminescent light.
Organic electroluminescence was first observed and studied in the 1960's U.S. Pat. No. 3,172,862 (Gurnee). In the 1980's, a double-layer structure for an OLED was disclosed by Tang (U.S. Pat. No. 4,356,429 (Tang); C. W. Tang et al., Appl. Phys. Lett. 51, 12: 913 (1987)). The discovery was based at least in part on employing a multilayer structure including an emitting layer and a hole transport layer of a suitable organic substrate. Alq3 (q=deprotonated 8-hydroxyquinolinyl) was chosen as the emitting material and was shown to provide relative advantages. For example, it may form relatively uniform thin films under 1000 Å using vacuum deposition. It is also a good charge carrier and it exhibits strong fluorescence. A conducting polymer-based OLED or PLED (polymer light-emitting device) was disclosed shortly after that by Friend at Cambridge University (Friend, WO Patent 90/13148 (Friend); U.S. Pat. No. 5,247,190 (Friend).
Since then, research on OLEDs and materials used in these devices has continued. OLED technology may be gaining marketplace acceptance, as suggested in a commercial report by Stanford Resources (http://www.stanfordresources.com), for example. OLEDs provide several advantages including: (1) low operating voltage; (2) thin, monolithic structure; (3) emitting light, rather than modulating light; (4) good luminous efficiency; (5) full color potential; and (6) high contrast and resolution. These advantages suggest possible use of OLEDs in flat panel displays.
One aspect related to the operation of an OLED is an organic luminophore or organometallic luminophore. An exciton, which includes a bound, excited electron and hole pair, may be generated inside an emitting layer (EL). If the exciton's electron and hole combine, a photon (visible light) may be emitted. To create excitons, an emitting layer (EL) may be sandwiched between electrodes of differing work functions. Electrons may be injected into one side from a metal cathode (e.g., Aluminium (Al), calcium (Ca), Magnesium-Silver alloy (Mg—Ag) are common cathode materials) via a electron transporting layer (ETL), while holes may be injected in the other side from an anode (e.g., Indium tin oxide (ITO) is a common transparent anode) via a hole transporting layer (HTL). The electron and hole may move into the emitting layer (EL) and may meet to form an exciton. An electroluminescent material in the emitting layer (EL) may be present in a separate emitting layer between the ETL and the HTL in what is referred as a multi-layer heterostructure. One possible embodiment of a basic heterostructure of an OLED is shown by a schematic diagram in FIG. 1, for example.
A major challenge in OLED manufacture is tuning a device such that a balancing number of holes and electrons meet in the emitting layer. This is difficult because, in an organic compound, the mobility of an electron is lower than that of a hole. In general, an exciton may be in one of two states, a singlet state (25%) or a triplet state (75%). Materials employed in an emissive layer are typically organic fluorophors, which emit light if a singlet exciton forms. However, by incorporating transition metals into a small-molecule OLED, the triplet and singlet states may be mixed by spin-orbit coupling, which may lead to emission from the triplet state. Triplet (phosphorescent) emitters can be four times more efficient than singlet emitters (S. R. Forrest et al., Nature 395: 151 (1998); H. ersin, Top. Curr. Chem. 241: 1 (2004)). In some cases, buffer layers and/or other functional layers may also be incorporated to improve the performance of the device. Likewise, OLEDs in which the electroluminescent emitters are the same materials that function either as an ETL or a HTL may be referred to here as single-layer heterostructures.
In addition to emissive materials that are present as the predominant component located between a hole transporting layer (HTL) and an electron transporting layer (ETL), another efficient luminescent material may be present in relatively low concentrations as a dopant in these layers to realize color tuning and efficiency improvement. If a dopant is present, the predominant material in the charge carrier layer may be referred to as a host. Materials as hosts and dopant may be matched so as to have a relatively high level of energy transfer from the host to the dopant, and to yield emission with a relatively narrow band centered near a selected spectral region with relatively high-efficiency and relatively high-brightness. The quantum efficiency of an electrofluorescence device is typically limited by the low theoretical ratio of singlet exciton (25%) compared to triplet exciton (75%) upon electron-hole recombination from electrical excitation. In contrast, if phosphorescent emitters are employed, the potentially for relatively high energy/electron transfer from a host to a phosphorescent emitters may result in improved electroluminescent efficiency (S. R. Forrest et al., Nature 395: 151 (1998); Y. G. Ma et al., Synth. Met. 94: 245 (1998)). Several phosphorescent OLED systems have been fabricated and have been demonstrated to be of relatively high-efficiency and relatively high-brightness.
OLEDs may be fabricated using materials that provide electrophosphorescent emission corresponding to one of the three primary colors, that is, red (R), green (G) and blue (B) so that they may be used as a component layer in full-color display devices, for example. Such materials may also be capable of being deposited as thin films using vacuum deposition techniques, which is a common method for OLED fabrication, so that the thickness of the emitting layer may be precisely controlled.
Improved methods of fabricating OLEDs and methods of making luminescent materials that may be employed in OLEDs continue to be sought.